1. Field of the Invention
This invention relates to a process for producing methacrolein in a high yield by oxidizing isobutylene or tertiary butanol with molecular oxygen with a specific catalyst.
2. Description of the Prior Art
Hitherto, many catalysts have been proposed for the gas phase catalytic oxidation of isobutylene or tertiary butanol. From the industrial point of view, however, there are yet many points to be improved.
For example, in the production of methacrolein, the yield of the product, methacrolein, based on the starting material (isobutylene or tertiary butanol) is still so low that the selectivity based on isobutylene is at most about 82-86% even with a catalyst. which has hitherto been called an excellent catalyst. Therefore, a catalyst system capable of giving a much higher selectivity is desired. Some of the catalysts containing molybdenum as a main component and additionally containing tellurium exhibit a considerably high yield in the early stage of the reaction. However, their activities drop markedly owing to structual change and scattering of tellurium, and there has been found substantially no catalyst enabling methacrolein to be obtained in a high yield for a long period of time.
When it is intended to produce methacrolein for use as a starting material for the production of a polymer, the methacrolein must be purified. When isobutyraldehyde is contained in the methacrolein as a by-product, the separation of the isobutyraldehyde from the methacrolein to purify the methacrolein is quite difficult because the boiling point of isobutyraldehyde (64.6.degree. C.) is very close to that of methacrolein (73.5.degree. C.). When methyl methacrylate is produced from methacrolein containing a large quantity of isobutyraldehyde, methyl isobutyrate is also formed simultaneously. It is known that if more than 100 ppm of methyl isobutyrate exists in methyl methacrylate, the resulting polymer is opacified or cracked. Therefore, the amount of isobutyraldehyde in the methacrolein used as the starting material for polymerization should be suppressed to 100 ppm or less.
Therefore, a catalyst system capable of suppressing the amount of the by-product, isobutyraldehyde, to 100 ppm or less is desired. When it is intended to use the resulting methacrolein as a starting material for producing an unsaturated ester directly from an unsaturated aldehyde, it is desired that the amount of by-products such as methacrylic acid and acetic acid is as small as possible. Thus, the use as an industrial catalyst involves a number of problems. The present inventors have conducted earnest and detailed studies with the aim of solving the above-mentioned problems. As a result, this invention has been accomplished.